טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentShay Hamer
SubjectResidual Strength of Carbon/Epoxy Laminates with MWCNT
Toughened Thermoplastic Interleaving
DepartmentDepartment of Materials Science and Engineering
Supervisors Dr. Sherman Dov
Professor Emeritus Siegmann Arnon (Deceased)
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Damage Tolerance of composite structures, defined as the ability to sustain damage and function safely, is an expanding research subject in academics and aviation industry. Typical composite structures are subjected to a variety of low velocity impact events during service that can generate barely visible damage with significant effects on subsequent performance. One of the important damage tolerance assessment techniques is the Compression After Impact (CAI) test which directly indicates the residual strength of a structure and indirectly used as a tool in investigating impact damage minimization. Among different methods considered in improving damage tolerance, thermoplastic interleaving is suggested in this investigation: Carbon/epoxy laminates interleaved with laboratory scale MWCNT reinforced electrospun Nylon 66 nanofibrilmat were implemented to improve the CAI. The results indicate that the epoxy-nanofibrilmat interface should be considered as an essential parameter influencing the CAI. Considering the impact damage mechanism comprises essentially a combined in plane mixed mode (Mode I and Mode II) interlaminar fracture, the inherent Mode I and Mode II fracture energy of the composite were evaluated. It was revealed that the nanofibrilmat was responsible for a major fracture energy improvement, in both modes I and II, and the effect of MWCNT addition was found to further improve by 20%. Comparable impact tests followed by CAI tests were conducted on three types of quassi-isotropic laminates, as follows; Non-interleaved laminates, laminates interleaved with 5% MWCNT reinforced NYLON66 nanofibrilmat and NYLON66 nanofibrilmat interleaved laminates. The damage projection done by an ultrasonic scanning of the post impacted laminates postulates that the interleaved laminates were 3 times damaged than the non-interleaved laminates. The decrease at the damage resistance of the interleaved laminates is attributed to the higher thickness of the interleaved laminate compared to the non-interleave laminate, leading to higher rigidity. Rigid laminates may have lower resistance to impact loading, causing larger delaminations compared to the non-interleaved laminate. Furthermore, limited electrospun nanofibrilmat-epoxy adhesion is assumed, which enables decreasing the interface strength or creating preliminary delaminations. Both results in larger post impact delamination area. Finally, comparable CAI tests were conducted on the impacted laminates. A 20% CAI strength decrease of the interleaved laminates compared to the non-interleaved laminates was revealed. Moreover, no CAI strength difference was indicated between the MWCNT reinforced and the nanofibrilmat interleaved laminates. The decrease of the CAI strength is attributed to larger buckling phenomena of the laminate as a result of larger delamination area.